396 HOST-PARASITE RELATIONSHIPS 



A test for how successful selections by the different routes have been is made by 

 comparing the resistances of the resulting strains. As Webster showed, the strains of 

 mice resistant to S. enteritidis administered by the gastric route were likewise resistant 

 to the organisms introduced subcutaneously, intraperitoneally, and intravenously. 

 Derived susceptible strains by the intrastomachal test route were likewise even more 

 susceptible to the organisms introduced by the other three routes. ^The degree of 

 progress toward resistance, although apparently less than that for susceptibility, was in 

 fact more significant since resistant, unselected strains of mice were difficult to find in 

 natural populations, whereas those which were as susceptible as or more so than the 

 selected strains were relatively easy to find. 



Sample mice of the strains derived by Webster have been bred in our laboratory 

 for some years. They may be compared directly with our S and RI lines developed 

 here at Ames. The S mice are the most resistant and were established by selections 

 based on the results of intraperitoneal inoculations of the test organism (see table 61). 

 The RI mice take their origin from the Webster resistant mice but have had their resist- 

 ance greatly increased through our selections based upon the intraperitoneal route of 

 inoculation. The K and C mice are two branches of the Webster selections for resistance 

 which were split from the main line on the basis of their resistance and suceptibility 

 to St. Louis encephalitis and louping ill. Similarly the N and Q lines represent strains 

 similarly separated from this established susceptible strain. There is some variation 

 between his two resistant lines and between his two susceptible lines, but the resistant 

 group are definitely more resistant than the susceptible group on the basis of the 

 intraperitoneal tests as they were on the intrastomachal tests. But these resistant 

 strains are not so highly resistant as those selected by the intraperitoneal route in the 

 first place. 



These results are important from the methodologic viewpoint in pointing the way 

 toward developing resistant and susceptible hosts to different diseases so that these 

 diseases may be analyzed for the host, gene-based characters which are significant 

 to the given diseases. They are also significant in indicating that, until a natural 

 barrier can be differentiated from the diluting effect of distance from the vital region 

 attacked by the particular disease, it is proper to assume that every cell in the body 

 is endowed with properties which are contributory to making the animal resistant 

 or susceptible to a disease. 



Further support for the significant part played by the cells of the whole body in 

 resistance to a specific disease comes in a study of X-ray irradiation effects on resistance 

 mechanisms of genetically differentiated strains of mice exposed to S. typhimurium. 

 The experimental design, although straightforward, is at the same time a little complex. 

 Details are found in three papers of Stadler and Gowen. 1264, 1265 ' 1265a X rays were 

 chosen as a means of altering the resistance of the cells in different regions of the 

 body. The experiment was designed as a factorial having four elements. Five inbred 

 strains of mice having known differences in X-ray and typhoid sensitivities were utilized 

 in comparable numbers. The numbers for the two sexes were balanced for each strain. 



